Interference pigments for preparing forgeryproof documents

ABSTRACT

Interference pigments based on multiply coated plateletlike silicatic substrates comprising 
     A) a first layer comprising a colorless or selectively absorbing metal oxide with a high refractive index, 
     B) a second, non-selectively absorbing, semitransparent layer consisting essentially of carbon, a metal or a metal oxide, and optionally 
     C) a third layer comprising a colorless or selectively absorbing metal oxide in combination with scattering color pigments which are complementary in color to the reflection color of the interference pigments are useful for preparing valuable documents that are forgeryproof.

The present invention relates to the use of specific interferencepigments combined with scattering color pigments which are complementaryin color to the reflection color of the interference pigments, forpreparing valuable documents that are forgeryproof, in particulardifficult or impossible to photocopy.

The forgeryproofing of valuable documents such as paper money, postagestamps, checks, tax stamps, check cards, rail or air tickets, creditcards, telephone cards, lottery tickets, share certificates, gifttokens, passes or identity cards is becoming more and more important inview of the introduction of evermore powerful color copiers and theattendant scope for forgery.

Forgeryproofness has in the past been pursued in various ways. Forinstance, the originals were marked using magnetic pigments orfluorescent dyes. However, these markings have the disadvantage thattheir presence is not immediately recognizable visually; they can onlybe detected using appropriate, more or less complex reading apparatus.The same is true of marking with IR or UV dyes or pigments.

Greater interest therefore pertains to marks which are sufficientlyreliably identifiable with the unaided, naked eye and make it easy todistinguish the original from the copy. In practice this kind of marktook the form of water marks--silvery bright metal strips which show upblack in the copy owing to the total reflection of the light--orholograms. However, the disadvantage of these marks is that they areeasy to imitate or, in the case of holograms, costly and prone todamage. The most recent addition to the ways of marking authenticity isthe use of simple interference pigments. However, applied atop alight-colored background, interference pigments are virtually invisibleand the marking is consequently easily overlooked.

Interference pigments in the conventional sense (described for examplein US-A-3 087 827) are platelet-like luster pigments consistingpredominantly of a mica substrate coated with a layer of highlyrefractive material, eg. rutile, zirconium dioxide or tin dioxide, in acertain thickness (in general >50 nm) to form thin platelet colors.Interference pigments have to be applied in an oriented state. However,the intensity of the color of interference color pigments is lowcompared with that of scattering color pigments. They have anangle-dependent reflection maximum. Interference pigments of this typehave for many years been used in cosmetics and plastics.

US-A-3 087 827 describes interference pigments coated with a highlyabsorbing carbon layer, but these pigments have never been used fordocuments.

EP-A-353 544 describes interference pigments which have a metal coatingas being suitable for security printing; however, these pigments are notused in combination with specific color pigments.

There have also been disclosed multi-layer pigments which exhibit adistinct angle-dependent color change and are prepared by pulverizingthin film security devices--thin multi-layer films detached from thesubstrate; cf. US-A-4 434 010, US-A-5 059 245. The optical principle ofsuch pigments is that of the transmission filter. The sequence of layersmaking up such pigments typically includes a dielectric layer of lowrefractive index, eg. silicon dioxide, a semitransparent reflectingmetal layer, eg. titanium, aluminum or chromium, a dielectric layer oflow refractive index, eg. silicon dioxide, a highly reflecting opaquemetal layer, eg. aluminum or titanium, a dielectric layer of lowrefractive index, eg. silicon dioxide, a semitransparent reflectingmetal layer, eg. aluminum, chromium or titanium, and a dielectric layerof low refractive index, eg. silicon dioxide.

Pigments of this type have the disadvantage that they are costly and notstable to alkali and, what is more, usually do not have the desired acidfastness.

It is an object of the present invention to provide suitable pigments orpigment combinations for the forgeryproof marking of valuable documents,so that in particular photocopying these documents with powerful copiersis made more difficult or, by virtue of hue falsification, impossible.

We have found that this object is achieved by the use of interferencepigments based on multiply coated plateletlike silicatic substratescomprising

A) a first layer comprising a colorless or selectively absorbing metaloxide with a high refractive index,

B) a second, non-selectively absorbing, semitransparent layer consistingessentially of carbon, a metal or a metal oxide, and optionally

C) a third layer comprising a colorless or selectively absorbing metaloxide in combination with scattering color pigments which arecomplementary in color to the reflection color of the interferencepigments, for preparing forgeryproof documents, in particular valuabledocuments that are difficult or impossible to copy.

The contemplated interference pigments are products known per se. Theyare described for example with methods of preparation in US-A-3 087 827,US-A-3 087 828 and the earlier German Patent Application P 41 41 069.

Suitable plateletlike silicatic substrates are in particularlight-colored or white micas, particularly preferably flakes of,preferably wet ground, muscovite. It is of course also possible to useother natural micas, such as phlogopite or biotite, artificial micas ortalc or glass flakes.

These substrates have already been coated with a first layer comprisinga colorless or selectively absorbing (i.e. non-black) metal oxide ofhigh refractive index, for example titanium oxide, zirconium oxide, tinoxide, chromium oxide, iron oxide, aluminum oxide, silicon oxide, zincoxide, bismuth oxychloride or mixtures thereof. Preference is given tocoatings with iron(III) oxide or zirconiumoxide and particularlypreferably with titanium dioxide,

These singly coated pigments are common knowledge; cf. for exampleDE-C-1 467 468, EP-A-45 851, DE-A-3 237 264 or DE-A-3 617 430. Metaloxide-coated mica platelets are also commercially available under thenames Iriodin (E. Merck, Darmstadt), Flonac® (Kemira Oy, Pori, Finland)or Mearlin® (Mearl Corporation, Ossining, N.Y.).

The second, non-selectively absorbing, transparent layer may be composedof carbon, metals, for example those which can be applied by gas phasedecomposition of volatile compounds, such as iron, cobalt, nickel,chromium, molybdenum or tungsten, or black metal oxides, such as ironoxide, magnetite, nickel oxide, cobalt oxides (CoO, Co₃ O₄), vanadiumoxides (VO₂, V₂, V₂ O₃) or mixtures thereof, in particular iron andmagnetite. The use of interference pigments coated essentially withcarbon is preferred.

The interference pigments used may additionally have a third layercomprising a colorless or selectively absorbing metal oxide. Suitablemetal oxides for this layer are for example titanium oxide, zirconiumoxide, tin oxide, chromium oxide, iron oxide, aluminum oxide, siliconoxide, zinc oxide or mixtures thereof. Particular preference is given toiron(III) oxide, titanium dioxide, zirconiumdioxide or mixtures thereof.The third layer of oxide is advantageously adapted in the individualcase to the second, black layer.

Preference is given to using interference pigments having 2 layers.

Preference is further given to using interference pigments which, basedon the weight of the luster pigment, contain from 0.03 to 5% by weight,preferably from 0.1 to 0.6% by weight, of a second non-selectivelyabsorbing layer. This corresponds to a thickness for the second layer offrom 1 to 30 nm.

Preference is further given to using interference pigments having anaverage particle size (based on the largest particle diameter) of from 5to 30 μm.

Scattering color pigments for the purposes of the present invention arepigmentary colorants which work chiefly by selective absorption ordiffuse scattering. As scattering color pigments it is also possible touse scattering white pigments surface-colored with transparentcolorants.

Suitable scattering color pigments include not only inorganic but alsoorganic pigments having a refractive index n greater than 1.65.

Examples of inorganic pigments are iron oxides, bismuth vanadate,colored spinels and nickel titanium yellow.

Examples of organic pigments are those of the class of the monoazopigments (eg. products derived from acetoacetarylide derivatives or fromβ-naphthol derivatives), laked monoazo dyes, such as lakedβ-hydroxynaphthoic dyes, disazo pigments, fused disazo pigments,isoindoline derivatives, derivatives of naphthalene- orperylene-tetracarboxylic acid, anthraquinone pigments, thioindigoderivatives, azomethine derivatives, quinacridones, dioxazines,pyrazoloquinazolones, phthalocyanine pigments or laked basic dyes, suchas laked triarylmethane dyes, provided they have a refractive index ngreater than 1.65. Otherwise these pigments are suitable for the surfacecoloring of scattering white pigments.

A suitable scattering white pigment for surface coloring is inparticular titanium dioxide.

The use of the interference pigments according to the invention has twoadvantages for visual perception. First, the pigmented marking is easyto see from every angle, even against a transparent or light-coloredground. Secondly, the coating has the effect that the reflectedinterference color can be seen significantly more clearly and with amuch higher intensity. Furthermore, coating valuable documents with theinterference pigments to be used according to the invention has theeffect that copying, even with powerful color copiers, gives rise tooff-shade colors which, combined with the color pigments according tothe invention become invisible.

On copying for example a document marked with a violet interferencepigment (structure: mica substrate/115 nm of rutile/20 nm of carbon) anda scattering white pigment (rutile), the interference pigment with theviolet reflection color will appear yellow against the white scatteringbackground. If the background is pigmented with iron red, cobalt spinelblue or halogenated phthalocyanine instead of titanium dioxide, thecolor copy will show brownish, greenish or yellow-green off-shadecolors. The same result is obtained on coloring the scattering whitepigment with dyes, for example paper dyes, in red, blue or yellow.

The combination according to the invention of the interference pigmentswith the scattering color pigments which are complementary in color tothe reflection color of the interference pigments is particularlyadvantageous on applying the interference pigments atop a background ofcolor pigment. If, for example, an interference pigment having a bluishviolet reflection color is applied atop a background colored with apigment that scatters yellow light, the color copy will show virtuallynothing other than yellow; that is, the bluish violet interferencepigment will be virtually invisible.

To obtain the above-described effects, the black absorbing layer of theinterference pigments must not be too thick. It must be semitransparentto incident light. Pigments in thick layers which totally absorbincident light show up black in color copies. When a pigment having ared, blue or green reflection color shows up as black in the copy, thatalso counts as an off-shade color.

Forgeryproof documents are preferably prepared using interferencepigments having a violet or blue reflection color. They are used incombination with scattering color pigments having a yellow or greenishyellow hue.

The above-described colorants are applied via the preparation of aprinting ink and subsequent printing using conventional printingmethods, eg. screen printing, intaglio printing, bronzing, flexographicprinting or offset printing.

Although the preparation of copyproof documents with interferencepigments and scattering color pigments is always best with theinterference pigments ending up at the very top, i.e. printed atop aprinted ground of scattering color pigment, the desired effect of huefalsification is still noticeable--albeit not as powerfully--when theinterference pigment and the scattering color pigment are mixed.

The Examples which follow illustrate the invention.

EXAMPLE 1

100 g of the commercial mica pigment Iriodin 9225 rutile pearl blue (E.Merck, Darmstadt; average particle size 19 μm, specific surface area 3m² /g muscovite, rutile coating) were introduced with stirring into asolution of 4 g of sorbitol in 100 ml of water. The pasty mass was driedat 160° C. in a drying cabinet for 10 h, then coarsely comminuted andheated over 70 min to 460° C. under nitrogen in a rotating sphere oven.The product was then maintained at that temperature for 3 h andthereafter cooled down under nitrogen.

The pigment obtained has a deep blue color and a carbon content of 0.5%by weight. Electron micrographs show that the mica particles have auniform coating of carbon. From the carbon content it is possible tocalculate a layer thickness of 2.5 nm.

The pigment was then applied by bronzing to a yellow sheet of paperprinted uniformly with a printing ink comprising a bismuth vanadatepigment. For this the yellow paper coated with bismuth vanadate pigmentwas printed with a commercial non-pigmented offset binder (composition:maleate resin/linseed oil/mineral oil) and then dusted with theabove-prepared blue luster pigment. The pigment adheres in thebinder-printed areas, whereas it is readily removed from the unprintedareas using a velvet roll.

The print prepared with the luster pigment has a shiny blue color. Acopy made with a Canon CLC 500 color copier shows nothing but yellow.The areas of the paper printed with the luster pigment are virtuallyinvisible in the copy.

EXAMPLE 2

Example 1 was repeated to coat 100 g of the commercial mica pigmentIriodin 9219 rutile pearl purple WR with carbon, which produced a deeplypurple-colored pigment, and applied by the bronzing technique to yellowpaper printed with bismuth vanadate.

The print prepared with the luster pigment has a shiny purple color. Acopy made with a color copier (Canon CLC 500) shows nothing but yellow.The areas printed with the luster pigment are virtually invisible in thecopy.

EXAMPLE 3

100 g of the commercial mica pigment Iriodin 231 rutile fine green wereintroduced with stirring into a solution of 8 g of sorbitol in 100 ml ofwater. The mixture was dried in a drying cabinet at 155° C., thendeagglomerated with a spatula, heated in a rotating sphere oven undernitrogen at 450° C. for 18 min, and cooled down under nitrogen to roomtemperature over 4 h.

The pigment obtained has a deep green reflection color. It has a carboncontent of 0.8% by weight. The coating of carbon is visible under theelectron microscope.

The pigment was stirred into a commercial alcoholic binder solution toform an intaglio printing ink containing 52% by weight ofnitrocellulose/ethanol/dioctyl phthalate and 55% by weight of pigment.

This ink was then printed (54 lines per cm 120°, well depth 40-45 μm)onto red coated paper coated with Fe₂ O₃ -containing offset ink.

What is claimed is:
 1. A process for preparing forgeryproof documentscomprising printing said documents with interference pigments based onmultiply coated platelet-shaped silicatic substrates comprising:A) afirst layer comprising a colorless or selectively absorbing metal oxidewith a high refractive index, B) a second, non-selectively absorbing,semitransparent layer consisting essentially of carbon, a metal or ametal oxide, and optionally C) a third layer comprising a colorless orselectively absorbing metal oxide in combination with scattering colorpigments which are complementary in color to the reflection color of theinterference pigments.
 2. The process as claimed in claim 1, wherein thelayer (B) consists essentially of carbon.
 3. The process as claimed inclaim 1, wherein the layer (B) is from 1 to 30 nm in thickness.
 4. Theprocess as claimed in claim 1, wherein the interference pigment has 2layers.
 5. A forgeryproof document printed by the process of claim 1.